The Grand Canyon in northwestern Arizona was formed by the erosive power of the weather and the Colorado River and its tributaries as they scoured away billion-year-old rocks. Although known to Native Americans for thousands of years, the vast gorge was not discovered by the first Spanish explorers until 1540. Grand Canyon National Park was established in 1919, preserving the more than 1.2 million acres of colorful cliffs and waterways that are home to 75 species of mammals, 50 species of reptiles and amphibians, 25 species of fish, and more than 300 species of birds. The canyon stretches 277 miles, with some sections reaching a mile deep and 18 miles across. More than five million visitors view the canyon annually, often hiking or riding mules down to the canyon floor, while the more adventurous opt for boating or rafting the Colorado River through the canyon.